Before you unhook your trailer from your tow vehicle use a piece of string or measuring tape with a small spring clip on it.
Measure the height from the ground to the hitch. Mark this length on the string or tape using your clip. When it's time to lower or raise your trailer to hitch it back up, you'll know exactly what height to return it to so you can back up your truck to the right hitch height without a lot of guesswork.